A worldwide humanitarian and military problem is the detection, localization, and subsequent neutralization of buried antipersonnel (AP) and antitank (AT) landmines (hereinafter simply referred to as landmines) as well as unexploded ordnance. The word “detection” is somewhat of a misnomer since it is not the detection of the landmines which is the difficulty but rather the discrimination between landmines and clutter. Clutter is not noise in the sense that is not simply an additive random component which is added to the desired signal. Clutter signals are generated by real targets such as roots, hubcaps, rocks, or other subsurface anomalies which individually are deterministic, but over the entire sample space are randomly distributed. The rejection of clutter is not improved by increases in power or other normal methods of signal to noise ratio (SNR) improvement such as noncoherent addition. Landmine detectors should be able to discriminate landmines from among the various types of clutter. In the case of antipersonnel (AP) mines in particular, their small size and reduced magnetic signatures make their discrimination from clutter particularly difficult. In addition to rocks, roots, and other sizeable detritus, soil anomalies such as hard layers of fragipan have mine-like characteristics which make them difficult to distinguish from landmines. Actual landmine signatures can differ significantly from class prototypes acquired through laborious data collection and processing due to variations in the type and moisture content of the soil in which they are buried, the existence of moisture which has seeped into the landmines themselves, as well as any explosive fill replacement material which has been used in the construction of dummy mines for testing.
Ground penetrating RADARs (GPR) have been used for landmine detection for some 20 years. At least at shallow depths, landmine detection is not a problem and a variety of signal processing techniques can be used to estimate the statistical characteristics of the background and subsequently subtract it from the data to improve detection and discrimination. Imaging ground penetrating RADAR has typically been limited to the extraction of external characteristics of a landmine and its orientation because of limited spatial resolution. What is needed is a landmine detection system capable of imaging the internal structure of non-metallic landmines and the detailed external structure of metallic landmines in order to distinguish them from clutter and, less importantly, from among other landmines.